The invention relates to a magnetic bearing apparatus for the journalling of a body, in particular of a rotor, in accordance with the preamble of the independent patent claim.
Magnetic bearing apparatuses for the non-contact journalling of bodies, such as for example rotors of electric motors, rotating shafts, pump rotors or non-rotating bodies, are increasingly gaining in importance. Bearing apparatuses of this kind typically comprise a stator, which is provided with at least one electrical winding by means of which a magnetic control field can be produced and regulated. By means of this regulatable control field, magnetic forces are exerted on the rotating or levitating body to be journalled which hold the latter without contact in a desired position relative to the stator.
A magnetic bearing is disclosed in WO-A-97/07340, the stator of which comprises a three-phase rotary current winding for the production of the magnetic control field. The radial magnetic bearing forces are produced through the superposition of a unipolar magnetic field on this rotary field.
In WO-A-95/18925 a special form of a magnetic bearing apparatus is disclosed, namely a so-called bearing-free motor. A bearing-free motor is an electromagnetic rotary drive in which the rotor is journalled with respect to the stator without contact by means of magnetic forces. The characteristic to which the bearing-free motor owes its name is that the stator is designed as a bearing and drive stator and the rotor both as a bearing and drive stator and as a drive rotor. This means that in a bearing-free motor the magnetic bearing and the drive device form an inseparable physical unit. Separate magnetic bearings or separate drive apparatuses for the rotor are not provided. As a consequence, the so-called bearing-free motor is also a magnetic bearing apparatus with an integrated rotary drive.
The bearing and drive stator of a bearing-free motor is designed or, respectively, provided with windings in such a manner that it produces an electromagnetic rotary field which, on the one hand, exerts a torque on the rotor which drives its rotation about the axis of rotation, and which, on the other hand, exerts a transverse force on the rotor which can be set as desired so that its radial position relative to a plane perpendicular to the axis of rotation can be predetermined or actively regulated respectively. For this the electrical windings of the stator--as for example has already been disclosed in the already cited WO-A-95/18925--comprises a drive winding with the number of pole pairs p and a control winding with the number of pole pairs p.+-.1 for the production of a magnetic control field. An electromagnetic rotary field can be produced with these two windings which exerts both a driving torque and also a transverse force which can be set as desired on the rotor. The rotor can thus be actively controlled or regulated respectively with respect to three degrees of freedom, namely the rotation about the axis of rotation A and its radial position relative to a plane perpendicular to the axis of rotation (two degrees of freedom). With respect to three further degrees of freedom, namely its axial deflection in the direction of the axis of rotation and tiltings relative to the plane perpendicular to the axis of rotation A (two degrees of freedom) the rotor is passively magnetically, that is, not controllably, stabilized by reluctance forces.
The term "bearing-free motor" is to be understood in this sense for the following explanations. Reference is made here to WO-A-95/18925 for further details of the design and especially the control or regulation respectively of the bearing-free motor.
In the bearing-free motor in accordance with WO-A-95/18925 the control winding for the production of the magnetic control field by means of which the radial position of the rotor is regulated can also be designed as a three-phase winding in a manner similar to that disclosed in WO-A-97/07340 for the magnetic bearing. In the last named publication several reasons are also mentioned why it is advantageous to design the control winding as a three-phase rotary current winding.
A problem with known magnetic bearing apparatuses is that when faults arise, such as for example the failure of an amplifier stage or the breakage of an electrical line in one of the phases of the control winding, a correct functioning of the bearing apparatus is no longer given. This can represent a safety hazard in particular in very sensitive applications, e.g. in blood pumps.